Semiconductor lasers having multi-quantum well active layers have become desirable because these lasers have several superior properties compared with conventional double heterostructure lasers. These superior properties include lower threshold currents, better threshold current temperature dependence, and shorter emission wavelengths. In order to utilize multi-quantum well lasers having a visible light wavelength, a stable transverse mode must be realized using a built-in refractive index optical waveguide. In order to obtain stable transverse mode oscillation, semiconductor lasers have been developed utilizing a self-aligned structure, i.e., a built-in optical and current confinement structure which stabilizes the transverse mode.
FIG. 5 shows a cross-sectional view of a multi-quantum well laser having a self-aligned structure as described in Applied Physics Letters Vol. 45, pp. 818-820 (1984). In FIG. 5, the self-aligned structure is characterized by a stripe shaped groove 10 produced by removing a central stripe portion of a highly absorbing n-type GaAs current blocking layer 5. A thin (0.2 .mu.m) p-type AlGaAs first upper cladding layer 4 is positioned between current blocking layer 5 and an active layer 3 having a multi-quantum well structure. As described in further detail below, this structure provides a built-in optical wavelength to stabilize the transverse mode. Additionally, this structure restricts current injection to the region within stripe shaped groove 10 because of the current blocking reverse biased pn junction formed between current blocking layer 5 and first upper cladding layer 4.
The self-aligned structure shown in FIG. 5 is produced by successively growing an n-type Al.sub.0.45 Ga.sub.0.55 As lower cladding layer 2, multi-quantum well active layer 3, p-type Al.sub.0.45 Ga.sub.0.55 As first upper cladding layer 4, and n-type GaAs current blocking layer 5 on an n-type GaAs substrate 1 by vapor phase epitaxy. Relative positional terms such as "upper" and "lower" are used herein not to limit the overall device to a particular orientation in space, but simply to distinguish between different layers. Thus, the lower cladding layer does not necessarily prohibit the laser from being operated in inverted configuration, but is simply to suggest that the lower cladding layer is the cladding layer closest to the substrate. After this first epitaxial growth process, stripe shaped groove 10 is produced by selectively etching a central stripe portion of current blocking layer 5 using conventional photolithography and etching techniques. Thereafter, a p-type Al.sub.0.45 Ga.sub.0.55 As second upper cladding layer 6 and a p-type GaAs contact layer 7 are grown in a second epitaxial growth process. Metal electrodes 8 and 9 are then disposed on the bottom surface of substrate 1 and on contact layer 7, respectively. Finally, the wafer upon which the lasers are grown is separated into chips, thereby completing the self-aligned structure laser devices.
In the multi-quantum well laser of FIG. 5, a pn junction exists between active layer 3 via p-type first upper cladding layer 4 and n-type lower cladding layer 2. As previously stated, the reverse biased current blocking pn junction existing between current blocking layer 5 and first upper cladding layer 4 confines current flow to the stripe shaped groove 10. Thus, stripe shaped groove 10 defines a channel-like region in active layer 3. When a forward bias voltage is applied to the pn junction across active layer 3 through metal electrodes 8 and 9, current flows concentratedly through stripe shaped groove 10 and carriers are injected into and confined to the channel-like region of active layer 3. When current above the threshold current level flows through active layer 3, carrier recombinations generate photons and lasing action is sustained. Because lower cladding layer 2 and first upper cladding layer 4 have a higher aluminum content than active layer 3, these layers have a lower index of refraction so that photons leaving active layer 3 are refracted back into it. Additionally, the cladding layers have a wider energy band gap which discourages carriers from overshooting active layer 3 after injection.
As stated previously, the self-aligned structure of FIG. 5 provides a built-in optical waveguide to stabilize the transverse mode. In this structure, the evanescent optical field penetrates into the highly absorbed n-type GaAs current blocking layer 5 through the thin first upper cladding layer 4 outside stripe shaped groove 10. Thus, the transverse mode is confined to stripe shaped groove 10. Current blocking layer 5 therefore provides an effective optical refraction differential in the multi-quantum well of active layer 3, so that the transverse mode is stabilized and photons are confined to the channel-like region of active layer 3. These photons oscillate between facets of the laser until they are ultimately ejected through one of the facets to produce a beam of coherent radiation.
In the prior art semiconductor laser of FIG. 5, when the wafers are cleaved to produce individual devices, mirrors are created at the device edges or facets which produce and enhance photon oscillation in the active layer to produce coherent radiation. However, as a result of the cleavage operation, crystal defects occur at the edges of the laser which absorb photons in the active layer near the mirrors. This optical absorption serves to increase the temperature at the laser facets and causes what is known as catastrophic optical damage. The possibility of catastrophic optical damage limits the maximum output power of semiconductor lasers. Additionally, the temperature increase due to optical absorption degrades the laser at the facets, reduces life and can ultimately destroy the laser.
In multi-quantum well lasers which are not of the self-aligned structure type, the semiconductor layers are typically segregated into those of one conductivity type above the active layer and the opposite conductivity type below the active layer. Impurities or other means are used to disorder the multi-quantum well along its edges to define a nondisordered central channel-like active layer region. Those impurities tend to reduce or prevent current injection in the disordered region, confining current flow in the active layer to the central channel-like region. The same mechanism is thus available for disordering the active layer to form window regions at the laser facets. This mechanism is effective both in confining current to the channel-like active layer region, and in creating window regions to prevent optical absorption at the facets.
To the extent the window regions are created by impurities of a given conductivity type in such lasers, that conductivity type can be coordinated with the segregated conductivity types above or below the active layer to reduce current leakage through the device. However, as can be seen in FIG. 5, the segregation of p-type and n-type regions in a self-aligned structure multi-quantum well laser is not as complete. More particularly, while it is possible to keep all of the layers below the active layer of a given conductivity type, such as n-type, the layers above the active layer must be of both conductivity types so that the current blocking layer can form a reverse biased junction with one of the upper cladding layers. This intermixing of n-type and p-type layers above the active layer can present difficulties in eliminating current leakage if impurities are introduced into the upper cladding layers for forming window regions as is conventional in the aforementioned prior art lasers.
Applicants believe it is for these reasons that when it was desired to achieve the high efficiency of the self-aligned structure multi-quantum well laser, power limitations due to catastrophic optical damage were accepted in order to avoid the possibility of current leakage which would decrease the efficiency of the device. While it might be technically possible to increase the power output of a self-aligned structure laser by introducing a window structure, the possibility of increased current leakage, resulting from the impurities needed for the window structure, led to a decrease in efficiency which made such an approach undesirable.